Method of making pulverulent chromium



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Patented July 31, 1934 v UNITED STATES PATENT OFFICE METHOD OF MAKINGPULVEEULENT CHROMIUM Leo D. Jensen, Chicago, 111.

No Drawing. Application November 27, 1931, Serial No. 577,707

7 Claims.

, acter, said deposit generally forming curly scales,

collecting said deposit, washing and drying it and comminuting; all asmore fully hereinafter set forth and as claimed.

Because of the great hardness and strength of the metal chromium,production of fine powdered or granular chromium has offered greattechnical difiiculties. Because of the high heat of oxidation ofchromium, it is not practical to reduce it from its oxid by gas currentsat a temperature below the melting point, as is done with various othermetals. Metallic chromium formed by the aluminothermic process isextremely hard and cannot be economically comminuted or granulated byordinary grinding machinery to give a powdered product. In granulatedchromium so made, the proportion of ments is usually great. As a matterof fact, in attempts of. this character there was sometimes more wear onthe crushing machinery than on the chromium. Electrically depositedchromium producedby the ordinary method is also of extreme hardness andis in thin films cohering to the metal used as the cathode. Sometimes,arborescent deposits, or trees", occur in ordinary plating methods, butthese are also extremely hard and it is not practical to pulverize themto form pure, finely divided chromium.

an the electroplating processes for the deposition of chromium now inuse employ a solution of @1103 as a bath, this solution containing minoramounts of sulfate and of trivalent chromium. Under proper conditions,very thin, extremely hard deposits can be obtained cohering to the metalplated; a fact which is responsible for the extensive use cl chromiumplating today. In this plating operation, it is known that betteremciency can be attained at temperatures cl 25 6. or below than athigher temperatures. In these plating processes, the current densitiesare ordinarily such. that there is a formation or hy drogensimultaneously with the chromium; but conditions are, oi course, sochosen that this iormation of gas does not interfere with the plathis.

I have found that by using high current densities, densities high.enough to cause a copious evolution of hydrogen, in lieu of forming acoherent continuous coating, I produce chromium as non-coherent depositswhich can be readily resteel, etc., from the grinding ele-g moved.Advantageously, the current density is high enough to produce curly,flaky non-adherent deposits, cracking and breaking away from thecathode. I am of the impression that as the layer on the cathodethickens, the tension of the last formed layer cracks and curls theunderlying layers. Chromium formed in this way, after removal from thebath, washing and drying, is of frangible character and can be readilycomminuted to a powder as fine as may be desired,

without great wear on the crushing apparatus. Whether this frangibilityis due to the formation of small independent crystals or is due toinclusion of hydrogen in the metal, I am not aware.

It may be due simply to unrelieved tensions in the layer.

The lower the temperature of the bath, the better is the currentefliciency and the quicker is the formation of frangible chromiumdeposits.

It is found in practice that there is a lower (5 limit of currentdensity varying with different cathode materials. At less densities, thecurling deposit is not formed. Cathodes of aluminum or aluminum alloyare particularly eiiicient for the present purposes. quired for theproduction of detachable deposits. Lead or other unattackable anodes maybe employed.' Current may be applied until a given bath is exhausted orthe bath may be replenished from time to time by additions of ClOs 5 inwell known manner. Ordinarily, however, I do not use the same bathwithout interruption for any length of time; this being partly becausethere is apt to be an accumulation of detached chromium which is slowlydissolved by the bath go with consequent waste.

In general the electrolysis is run under conditime so as to formcurling, readily frangible flakes carried by the cathode, the cathodebeing removed before the flakes become thick enough cm to rendercrushing too expensive.

in a specific embodiment of the present invention, a bath was made bydissolving commercial (3103 to form a solution containing 400 grams perliter. This was acidified by addition of ordi- 1W nary commercialconcentrated sulfuric. acid until the total H2804 content of the bathwas 4 grams per liter. In this particular work, an aluminum cathode withabout 500 square inches surface area was used, being placed in closeproximity to lead anodes. The bath was made cold and kept at atemperature below 25 C.'by cooling coils. A current of 2,000 amperes waspassed between the electrodes for an hour when a curly non-coherentdeposit was formed. At this time, no

A certain time is rean occurring in the bath was also collected. The

' venient crushing device.

chromium was washed and dried and then crushed to a powder. In thisparticular work a ball mill was used.

Chromium made in the described manner is frangible enough to permitthe-use'oi any con- The limiting current density varies somewhat withthe cathodematerial and with the temperature, but using aluminumcathodes and a temperature below 25 C., it is of the order of 2 amperesper square inch.

The temperature or the bath is not specially important except as itcontrols efllciency. Good flaked deposits can be obtained attemperatures as high as 48 C. on nickel, aluminum and aluminum alloys.

Qther chromium plating baths than solutions of CrO; can be used; but thelatter are the best at present known.

As a rule, the flaked chromium is washed and dried prior to\comminuting, but it is sometimes convenient to wet grind the flakes toform a paste or pulp and afterwards dry this to produce the desiredpulverulent chromium.

The present method may be applied to making granular chromium orpulverulent chromium of any mesh desired. The flake' chromium issometimes utilized as such.

The product produced according to my invention has many uses, forexample, in the manutacture oi. inks, paints, coatings, alloys, etc.

It is to be understood that the invention is not limited to any specificset oi. conditions described above, but may be more or less materiallymodifled without departure from the spirit and scope of the same ascalled for in the following claims.

What I claim is:

1. A process of producing metallic chromium as a flne powder whichcomprises electrolyzing a chromium plating bath containing a highconcentrationoi' ClOs with a cathode made of a metal-not attacked by thebath, a high current density on said cathode with temperature and otherconditions producing a copious evolution of hydrogen gas and adeposition of chromium upon the cathode in non-adherent brittle flakes,detaching said flakes irom the cathode and grinding the flakes to apowder.

2. The process of claim 1 wherein the electrodeposition is efl'ected onan aluminum cathode.

3. The process oi. claim 1 wherein a current density above 2 amperes persquare inch of cathode is employed.

4. In the process 01' claim 1, maintenance of the plating bath at atemperature not above C.

5. In the production of pulverulent or granular chromium, the processwhich comprises electrolyzing a bath 01' C103 at a low temperature andat a high cathode current density giving metallic chromium in detachableflakes at the cathode with a copious evolution of hydrogen gas, washingand drying the detached flakes and comminuting to the flneness desired.

6. In the production 01' readily crushable chromium, the process whichcomprises electrolyzing a bath of CrO: with an aluminum cathode at atemperature not above 48 C. and at a sumciently high cathode currentdensity to give metallic chromium in detachable flakes with a com-- ousevolution of hydrogen.

7. In the production of readily crushable chromium, the process whichcomprises electrolyzing a bath of CrOa with an aluminum cathode at atemperature not above 48 C. and at a sufllciently high cathode currentdensity to give metallic chromium in detachable flakes with a copiousevolution 01' hydrogen, and washing and drying the detached flakes.

LEO D. JENSEN.

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